Positive electrodes for lithium-ion secondary batteries

Abstract

To provide means capable of improving an output characteristic in a lithium ion secondary battery.SOLUTION: A positive electrode for a lithium ion secondary battery includes a positive electrode active material layer including a positive electrode active material; a binder; and a conductive assistant. A value of a roughness factor defined as a value of a rate of a BET ratio surface area against a geometrical ratio surface area calculated from a mean particle diameter in the positive electrode active material is 3 or less. The binder is vinylidene fluoride- fluoride-hexafluoropropylene copolymer. At the time, a rate of a construction unit derived from fluoride-hexafluoropropylene against all of the construction unit numbers of the copolymer is 5 mol% or more. The content of the binder is greater than 0.31 mass% and is less than 4.85 mass% with respect to a total solid of the positive electrode active material layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium ion secondary battery. [Background technology]

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other applications, which are key to the widespread use of these electric vehicles. Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, are attracting attention for their high energy density and high output.

[0003] Patent Document 1 discloses a technology aimed at improving the safety of lithium-ion secondary batteries. Specifically, the technology described in Patent Document 1 is characterized by using a composite PTC (Positive Temperature Coefficient) binder containing polyvinylidene fluoride (PVDF) and a PTC (Positive Temperature Coefficient) function-imparting component (e.g., PVDF-HFP) as a binder for the positive electrode active material layer. As a result, when the temperature of the lithium-ion secondary battery rises, the binder melts, increasing the resistance of the positive electrode active material layer and suppressing heat generation in the battery. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 128139 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the inventors of the present invention have conducted research and found that the lithium ion secondary battery using the technology described in Patent Document 1 has a problem in that the output characteristics are insufficient.

[0006] Therefore, an object of the present invention is to provide a means for improving the output characteristics of a lithium ion secondary battery. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems, and in the process have found that the output characteristics of a lithium-ion secondary battery can be improved by controlling the roughness factor of the positive electrode active material contained in the positive electrode active material layer and the type and content of the binder within specific ranges, which has led to the completion of the present invention.

[0008] That is, one aspect of the present invention provides a positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. The positive electrode is characterized in that the positive electrode active material has a roughness factor of 3 or less, defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size of the positive electrode active material; the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and the proportion of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more; and the content of the binder is more than 0.31 mass% and less than 4.85 mass% of the total solid content of the positive electrode active material layer. [Effects of the Invention]

[0009] According to the present invention, it is possible to improve the output characteristics of a lithium ion secondary battery. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view schematically illustrating a stacked (flat) non-bipolar (internal parallel connection) secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 1 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] One aspect of the present invention relates to a positive electrode for a lithium-ion secondary battery (hereinafter simply referred to as "positive electrode") having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. This aspect is characterized in that the positive electrode active material has a roughness factor of 3 or less, defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size; the binder is a vinylidene fluoride-hexafluoropropylene copolymer (hereinafter also referred to as "PVDF-HFP"), and the ratio of the number of structural units derived from hexafluoropropylene (hereinafter also referred to as "HFP") to the total number of structural units of the copolymer is 5 mol% or more; and the content of the binder is more than 0.31 mass% and less than 4.85 mass% of the total solid content of the positive electrode active material layer.

[0012] The positive electrode for a lithium ion secondary battery according to the present embodiment can improve the output characteristics of the lithium ion secondary battery. The present inventors speculate that the mechanism by which this effect is achieved is as follows.

[0013] In this embodiment, a positive electrode active material having a roughness factor value of 3 or less has a small surface area. Furthermore, PVDF-HFP, which has a ratio of HFP-derived structural units (hereinafter simply referred to as "HFP ratio") of 5 mol % or more as a binder, is a gel-forming polymer and has higher flexibility than PVDF, a homopolymer of vinylidene fluoride, or PVDF-HFP having an HFP ratio of less than 5 mol %. By using such a positive electrode active material in combination with a binder, the binder is more uniformly coated on the surface of the positive electrode active material than in conventional positive electrodes. Furthermore, the uniform coating can be maintained even after repeated contraction and expansion of the positive electrode active material due to charge and discharge. Furthermore, PVDF-HFP with a high HFP ratio as described above can retain an electrolyte. Therefore, good contact between the surface of the positive electrode active material and the electrolyte is achieved, improving lithium ion conductivity. As a result, applying the positive electrode for a lithium ion secondary battery according to this embodiment to a lithium ion secondary battery can improve the output characteristics of the lithium ion secondary battery.

[0014] On the other hand, as shown in Comparative Examples 3 to 5 below, when a cathode active material with a roughness factor greater than 3 is used, or as shown in Comparative Examples 7 to 9 and 11 below, when PVDF is used or PVDF-HFP with an HFP ratio of less than 5 mol % is used, the output characteristics are insufficient. This is thought to be due to the large surface area of the cathode active material and the low flexibility of the binder, which makes it difficult to uniformly coat the surface of the cathode active material with the binder, or the binder's low electrolyte retention. In this state, repeated contraction and expansion of the cathode active material due to charge and discharge creates voids on the surface of the cathode active material where no electrolyte is present, reducing lithium ion conductivity and preventing the battery from exhibiting sufficient output characteristics.

[0015] The following describes the above-mentioned embodiment of the present invention with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0016] FIG. 1 is a cross-sectional view that schematically illustrates a flat (stacked) non-bipolar (internal parallel connection) secondary battery (hereinafter also simply referred to as a "stacked secondary battery") according to one embodiment of the present invention.

[0017] 1, the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power generating element 21 has a configuration in which a positive electrode in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 12 are laminated. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween.

[0018] As a result, the positive electrode, electrolyte layer, and negative electrode constitute one cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power generating element 21, active material layers may be disposed on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive electrode and negative electrode from FIG. 1, the outermost negative electrode current collectors may be located on both outermost layers of the power generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

[0019] A positive electrode current collector 25 and a negative electrode current collector 27, which are electrically connected to the electrodes (positive and negative electrodes), are attached to the positive electrode current collector 11′ and the negative electrode current collector 12, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and led out of the laminate film 29. The positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11′ and the negative electrode current collector 12 of the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary.

[0020] Fig. 2 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. Bipolar secondary battery 10b shown in Fig. 2 has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually occur, is sealed inside a laminate film 29, which is a battery exterior. In this specification, a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar secondary battery," and an electrode for a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar electrode."

[0021] As shown in FIG. 2 , the power generating element 21 of the bipolar secondary battery 10b of this embodiment has a plurality of bipolar electrodes 23, each having a positive electrode active material layer 13 electrically coupled to one surface of a current collector 11 and a negative electrode active material layer 15 electrically coupled to the other surface of the current collector 11. The bipolar electrodes 23 are stacked with an electrolyte layer 17 interposed between them to form the power generating element 21. The bipolar electrodes 23 and the electrolyte layers 17 are alternately stacked such that the positive electrode active material layer 13 of one bipolar electrode 23 faces the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the first bipolar electrode 23 with the electrolyte layer 17 interposed therebetween. That is, the electrolyte layer 17 is sandwiched between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the first bipolar electrode 23.

[0022] Adjacent positive electrode active material layers 13, electrolyte layers 17, and negative electrode active material layers 15 constitute a single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a configuration in which the cell layers 19 are stacked. In addition, a seal portion (insulating layer) 31 is disposed on the outer periphery of the cell layer 19. This prevents a liquid junction due to leakage of the electrolyte solution from the electrolyte layer 17, and prevents contact between adjacent current collectors 11 within the battery and short circuits caused by slight misalignment of the edges of the cell layers 19 in the power-generating element 21. The positive electrode side outermost current collector 11a, which is the outermost layer of the power-generating element 21, has the positive electrode active material layer 13 formed on only one surface. The negative electrode side outermost current collector 11b, which is the outermost layer of the power-generating element 21, has the negative electrode active material layer 15 formed on only one surface.

[0023] 2, a positive electrode current collector (positive electrode tab) 25 is disposed adjacent to the outermost current collector 11a on the positive electrode side, and extends from the laminate film 29, which is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is disposed adjacent to the outermost current collector 11b on the negative electrode side, and similarly extends from the laminate film 29.

[0024] The number of times that cell layers 19 are stacked is adjusted according to the desired voltage. In addition, in bipolar secondary battery 10b, the number of times that cell layers 19 are stacked may be reduced as long as sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In bipolar secondary battery 10b as well, it is preferable to use a structure in which power generating element 21 is vacuum-encapsulated in laminate film 29, which is the battery exterior, and positive electrode current collector 25 and negative electrode current collector 27 are exposed to the outside of laminate film 29, in order to protect against external impacts and environmental deterioration during use.

[0025] The main components of the positive electrode for a lithium ion secondary battery according to this embodiment will be described below. The positive electrode for a lithium ion secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive additive. The positive electrode active material layer is formed on the surface of an optional current collector.

[0026] [Current collector] The current collector has a function of mediating the transfer of electrons from the positive electrode active material layer and the negative electrode active material layer described later. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

[0027] Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Furthermore, foils in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

[0028] The latter conductive resin may be a resin in which a conductive filler is added to a non-conductive polymer material.

[0029] Examples of non-conductive polymer materials include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and polystyrene (PS).

[0030] The conductive filler can be any material that is conductive. Examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. While there are no particular limitations on the metal, it is preferable to use at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing such a metal. Furthermore, there are no particular limitations on the conductive carbon. Preferably, the conductive carbon contains at least one selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerenes.

[0031] The amount of conductive filler added is not particularly limited as long as it is an amount that can impart sufficient conductivity to the current collector, and is generally about 5 to 80 mass %.

[0032] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector includes at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the positive electrode active material layer and the negative electrode active material layer described later are electrically conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate member from these electrode active material layers. In such a configuration, the positive electrode active material layer described later constitutes the negative electrode, and the negative electrode active material layer described later constitutes the positive electrode.

[0033] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material, a binder, and a conductive additive, and may further contain optional components such as an electrolytic solution (liquid electrolyte).

[0034] (Cathode active material) The positive electrode active material has the function of releasing ions such as lithium ions during charging and absorbing ions such as lithium ions during discharging. In the positive electrode for a lithium ion secondary battery of this embodiment, the type of positive electrode active material is not particularly limited, but it is preferable that it be made of a space group R3m because it has a higher capacity. Positive electrode active materials belonging to the space group R3m have a layered structure (layered rock salt structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Therefore, the use of such a positive electrode active material can improve the battery capacity of a lithium ion secondary battery.

[0035] Examples of positive electrode active materials belonging to the space group R3m include lithium-transition metal composite oxides such as LiCoO2, LiNiO2, LiMnO2, Li(Ni-Mn-Co)O2, and Li(Ni-Co-Al)O2, in which a portion of the transition metal is substituted with another element. In some cases, two or more positive electrode active materials may be used in combination. A composite oxide containing lithium and nickel is more preferred, and Li(Ni-Mn-Co)O2 and a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NMC composite oxide") or Li(Ni-Co-Al)O2 and a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NCA composite oxide") is even more preferred, with NMC composite oxide being particularly preferred. That is, according to a preferred embodiment of the present invention, the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure. NMC composite oxides and NCA composite oxides have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are stacked alternately with oxygen atomic layers interposed between them. They contain one Li atom per transition metal M atom, and the amount of Li that can be extracted is twice that of spinel-type lithium manganese oxides, meaning that the supply capacity is doubled, resulting in high capacity.

[0036] As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which a portion of the transition metal element is replaced with another metal element. In this case, the other element may be Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, or Zn. Preferably, Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, or Cr. More preferably, Ti, Zr, P, Al, Mg, or Cr. From the viewpoint of improving cycle performance, Ti, Zr, Al, Mg, or Cr is even more preferable. However, the other metal element that can replace the transition metal element of the NCA composite oxide is one other than Al.

[0037] Since the NMC composite oxide has a high theoretical discharge capacity, it is preferable to use the NMC composite oxide represented by the general formula (1): Li aNi b Mn c Co d M x O₂ (wherein, in the formula, a, b, c, d, and x satisfy 0.9 ≤ a ≤ 1.2, 0 < b < 1, 0 < c ≤ 0.5, 0 < d ≤ 0.5, 0 ≤ x ≤ 0.3, and b + c + d + x = 1. M is at least one element selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. From the perspective of cycle characteristics, in the general formula (1), it is preferable that 0.4 ≤ b ≤ 0.92. Note that the composition of each element can be measured, for example, by inductively coupled plasma (ICP) optical emission spectrometry.) has the composition represented by.

[0038] Generally, nickel (Ni), cobalt (Co), and manganese (Mn) are known to contribute to capacity and output characteristics from the perspectives of improving the purity of the material and improving electron conductivity. Ti, etc. are those that partially substitute transition metals in the crystal lattice. From the perspective of cycle characteristics, a part of the transition metal element may be substituted by other metal elements. Since the crystal structure is stabilized by the solid solution of at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr, as a result, it is considered that the capacity degradation of the battery can be prevented even when charge and discharge are repeated, and excellent cycle characteristics can be realized.

[0039] As a more preferred embodiment, in the general formula (1), from the perspective of improving the balance between capacity and life characteristics, it is preferable that b, c, and d satisfy 0.44 ≤ b ≤ 0.92, 0.05 ≤ c ≤ 0.31, and 0.03 ≤ d ≤ 0.26. For example, LiNi 0.5 Mn 0.3 Co 0.2 O₂ is LiCoO₂, LiMn₂O₄, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3Compared to O2, etc., it has a larger capacity per unit mass and can improve energy density, which has the advantage of allowing the creation of compact, high-capacity batteries, and is also preferable from the perspective of driving range. 0.8 Co 0.1 Al 0.1 O2 and LiNi 0.8 Mn 0.1 Co 0.1 O2, LiNi 0.88 Mn 0.06 Co 0.06 O2, LiNi 0.86 Mn 0.08 Co 0.06 O2 is more favorable. On the other hand, LiNi 0.5 Mn 0.3 Co 0.2 O2 is LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 It has excellent lifespan characteristics comparable to O2.

[0040] The positive electrode for a lithium ion secondary battery according to this embodiment is characterized in that the roughness factor of the positive electrode active material is 3 or less. Here, the "roughness factor" is a parameter that indicates the surface smoothness of the positive electrode active material. It is calculated as the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle size" of the positive electrode active material (=BET specific surface area / geometric specific surface area) by the method described in the Examples section below. According to this embodiment, the roughness factor is controlled to 3 or less, so that the binder described below can be uniformly coated on the surface of the positive electrode active material. Since the binder has the function of retaining the electrolyte, it is believed that uniformly coating the binder improves the lithium ion conductivity near the surface of the positive electrode active material. As a result, according to this embodiment, the output characteristics of the lithium ion secondary battery can be improved. The roughness factor value is preferably 2.7 or less, more preferably 2.4 or less. Meanwhile, the lower limit of the roughness factor is 1 or more. When the positive electrode active material is a mixture of two or more materials, the BET specific surface area and the geometric specific surface area of each positive electrode active material constituting the mixture are measured by the above-mentioned method, and the BET specific surface area and the geometric specific surface area of the mixture are calculated by adding the obtained values by the mixing ratio.The roughness factor is then calculated from these calculated values by the same method as above.

[0041] There are no particular limitations on the method for controlling the roughness factor of the positive electrode active material to 3 or less, and conventionally known knowledge that can control the surface smoothness of positive electrode active material particles can be appropriately referred to. For example, as an example of a method for producing an NMC composite oxide, first, (1) a precursor (Ni x Co y Mn z)(OH)2 is synthesized by coprecipitation. Specifically, an aqueous solution containing raw materials such as nickel sulfate, cobalt sulfate, manganese sulfate, sodium hydroxide, and ammonium hydroxide in the desired stoichiometric ratio is stirred for a predetermined time. The precipitate is separated by filtration and then dried at a temperature of about 80°C for 8 to 15 hours to obtain a precursor. Next, (2) the obtained precursor is calcined at a temperature of about 600 to 800°C for 3 to 7 hours to obtain Ni x Co y Mn z O2 is obtained. (3) Then, (4) a lithium salt (e.g., lithium carbonate) is added to the obtained oxide in a 1-1.2 stoichiometric ratio, followed by pulverization and mixing in a ball mill. (4) Then, (5) the powder is calcined in air or an oxygen atmosphere at a temperature of approximately 600-800°C for approximately 3-5 hours, and then further calcined at a temperature of approximately 750-1000°C for approximately 2-12 hours. (5) Finally, (6) the calcined powder is washed with water to remove residual lithium salt, and then dried at approximately 80°C for approximately 10-15 hours to obtain an NMC composite oxide. (6) If necessary, the sample may be pulverized in a ball mill and then re-calcined in air or an oxygen atmosphere at 500-1000°C for 3-5 hours. The roughness factor of the NMC composite oxide can be controlled by adjusting the calcination temperature and time in (2), the degree of pulverization in (3), the calcination time and temperature in (4), and the degree of pulverization in (6). The primary particle size can be controlled by adjusting the calcination temperature and time in (2), the degree of pulverization in (3), the calcination time and temperature in (4), and the degree of pulverization in (6). Furthermore, by carrying out the re-firing (6) for a long time and at a high temperature, the surface after grinding becomes smoother and the roughness factor can be reduced.

[0042] Furthermore, as described above, methods for controlling the crystallite diameter of the positive electrode active material to a large value, for example, 1 μm or more, or for controlling the proportion of particles consisting of a single crystallite among the particles constituting the positive electrode active material to the above-mentioned preferred range can be found in patent documents such as Japanese Patent No. 6574222 and Japanese Patent No. 5702289, and non-patent documents such as Solid State Ionics, Volume 345, February 2020, 115200, and Journal of the Electrochemical Society, 165(5)A1038-A1045(2018).

[0043] In the positive electrode for a lithium ion secondary battery according to this embodiment, the positive electrode active material preferably satisfies Y / X≦70, where X (nm) is the crystallite diameter calculated by the Williamson-Hall method and Y (nm) is the average particle diameter (D50) calculated by laser diffraction. Here, Y / X is an index of the number of crystal grains constituting the positive electrode active material particles, and a smaller value indicates fewer crystal grains constituting the particles. The crystallite diameter X (nm) and the average particle diameter (D50) can be measured by the methods described in the Examples section below. Y / X is more preferably 50 or less, and even more preferably 40 or less. Meanwhile, the lower limit of Y / X is 1 or more. When Y / X is within the above range, the binder described below can be more uniformly coated on the surface of the positive electrode active material. Because the binder has the function of retaining the electrolyte, more uniform coating of the binder is thought to further improve the lithium ion conductivity near the surface of the positive electrode active material. As a result, the output characteristics of a lithium ion secondary battery using the positive electrode according to this embodiment can be further improved.

[0044] In the positive electrode for a lithium ion secondary battery according to this embodiment, the positive electrode active material preferably has the aforementioned R3m space group and, in addition, a peak intensity ratio ((003) / (104)) of the diffraction peak of the (003) plane to the diffraction peak of the (104) plane obtained by X-ray diffraction measurement is preferably 1.35 or more. The peak intensity ratio ((003) / (104)) is more preferably 1.4 or more, even more preferably 1.45 or more, and particularly preferably 1.48 or more. The upper limit of the peak intensity ratio ((003) / (104)) is not particularly limited, but is preferably 2.1 or less. The peak intensity ratio is an index of the crystallinity of the positive electrode active material, and a larger peak intensity ratio indicates higher crystallinity. A peak intensity ratio within the above range reduces defects within the crystal, thereby suppressing a decrease in battery charge / discharge capacity and durability. The peak intensity ratio can be controlled by the raw materials, composition, firing conditions, etc. The crystal structure and peak intensity ratio of the positive electrode active material can be determined by the measurement methods described in the Examples below.

[0045] In the positive electrode for a lithium ion secondary battery according to this embodiment, the tap density of the positive electrode active material is 2 g / cm 3 It is preferable that the concentration is 1.9 g / cm or less. 3 More preferably, it is 1.8 g / cm or less. 3 The lower limit of the tap density is not particularly limited, but is preferably 1.2 g / cm. 3 That's all. The smaller the tap density value, the smaller the volume of the space in the unevenness of the particle surface and the volume of the gaps between particles. When the tap density is within the above range, the binder described below can be more uniformly coated on the surface of the positive electrode active material. Since the binder has the function of retaining the electrolyte, it is thought that by coating the binder more uniformly, the lithium ion conductivity near the surface of the positive electrode active material can be further improved. As a result, it is possible to further improve the output characteristics of a lithium ion secondary battery using the positive electrode according to this embodiment.

[0046] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably within the range of 60 to 99 mass %, and more preferably within the range of 80 to 98 mass %, for example.

[0047] (binder) The binder (binding agent) functions to maintain the structure of the positive electrode active material layer by binding together the components contained in the positive electrode active material layer. In the positive electrode for a lithium ion secondary battery according to this embodiment, the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and the ratio of the number of hexafluoropropylene-derived structural units to the total number of structural units of the copolymer is 5 mol% or more. This ratio is preferably 6 mol% to 20 mol%, more preferably 6.5 mol% to 10 mol%, and even more preferably 6.9 mol% to 8 mol%. If this ratio is less than 5 mol%, the flexibility of the binder and its swelling in the electrolyte may be insufficient, which may result in an inability to uniformly coat the surface of the positive electrode active material with the binder or a reduced amount of electrolyte retention. As a result, the lithium ion conductivity near the surface of the positive electrode active material may decrease, potentially preventing sufficient output characteristics from being exhibited.

[0048] Furthermore, in the positive electrode for a lithium ion secondary battery according to this embodiment, the binder content is more than 0.31% by mass and less than 4.85% by mass relative to the total solid content of the positive electrode active material layer. The binder content is preferably 0.50% by mass or more and 2.97% by mass or less, and more preferably 1.01% by mass or more and 2.00% by mass or less. If the binder content is 0.31% by mass or less, the binding strength is insufficient, and the electrode structure may not be maintained. On the other hand, if the binder content is 4.85% by mass or more, the binder may not cover the surface of the positive electrode active material uniformly, and the effect of improving output characteristics may not be obtained.

[0049] (Conductive additive) The conductive additive has the function of forming an electron conduction path (conductive passage) in the positive electrode active material layer. When such an electron conduction path is formed in the positive electrode active material layer, the internal resistance of the battery is reduced and the output characteristics at high rates can be improved. In the positive electrode for a lithium ion secondary battery according to this embodiment, the surface of the positive electrode active material is uniformly coated with a binder, so that the conductive additive is essential from the viewpoint of ensuring an electron conduction path between adjacent positive electrode active materials and between the current collector and the positive electrode active material.

[0050] Examples of conductive additives include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), and fibrous carbon materials such as carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. One type of conductive additive may be used alone, or two or more types may be used in combination. Among these, fibrous carbon materials are preferred as the conductive additive, and carbon nanotubes are more preferred, as they are capable of forming a good electron conduction path.

[0051] The content of the conductive additive in the positive electrode active material layer (the total amount when two or more types are included) is preferably 2% by mass or less, and more preferably 1% by mass or less, relative to 100% by mass of the total solid content of the positive electrode active material layer. At such an upper limit, aggregation of the conductive additives is suppressed, resulting in favorable formation of an electron conduction path, thereby further improving the output characteristics. It is also possible to further improve the energy density of the lithium-ion secondary battery. While the lower limit of the content of the conductive additive is not particularly limited, it is preferably greater than 0% by mass, and is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. At such a lower limit, sufficient conductive additive is present to form an electron conduction path, thereby further improving the output characteristics.

[0052] In the positive electrode for a lithium ion secondary battery of this embodiment, the thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referred to as appropriate. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm. The thicker the positive electrode active material layer, the more positive electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the positive electrode active material layer, the more the discharge rate characteristics can be improved.

[0053] The porosity of the positive electrode active material layer is preferably 20 to 50%, more preferably 20 to 45%. When the porosity of the positive electrode active material layer is within the above range, contact between the electron conductive materials (conductive additive, positive electrode active material, etc.) in the positive electrode active material layer can be sufficiently maintained, preventing an increase in electron transfer resistance. Furthermore, since a sufficient amount of electrolyte exists between the positive electrode active material particles, an increase in lithium ion transfer resistance can be prevented. As a result, the output characteristics of the lithium ion secondary battery can be further improved.

[0054] The density of the positive electrode active material layer is preferably 2.10 to 3.00 g / cm 3 and more preferably 2.15 to 2.85 g / cm 3 and more preferably 2.20 to 2.80 g / cm 3 When the density of the positive electrode active material layer is equal to or greater than the above-mentioned lower limit, a battery having sufficient energy density can be obtained. On the other hand, when the density of the positive electrode active material layer is equal to or less than the above-mentioned upper limit, a sufficient amount of electrolyte is secured to fill the voids, and an increase in lithium ion migration resistance in the positive electrode active material layer can be prevented. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery. In this specification, the density of the positive electrode active material layer is measured by the following method.

[0055] The density of the positive electrode active material layer is calculated according to the following formula. Cathode active material layer density (g / cm 3 ) = mass of solid material (g) ÷ volume of positive electrode active material layer (cm3 ).

[0056] The mass of the solid material is calculated by adding up only the mass of the solid material among the masses of the materials in the positive electrode active material layer, and the volume of the positive electrode active material layer is calculated from the thickness and application area of the positive electrode active material layer.

[0057] The positive electrode for a lithium ion secondary battery according to this embodiment can improve the output characteristics of the lithium ion secondary battery by being applied to the lithium ion secondary battery. Therefore, according to another embodiment of the present invention, there is provided a lithium ion secondary battery including a power generating element having the positive electrode for a lithium ion secondary battery according to the above-described embodiment of the present invention. The components of the lithium ion secondary battery other than the positive electrode will be briefly described below.

[0058] [Negative electrode active material layer] (Negative electrode active material) The negative electrode active material has the function of releasing ions such as lithium ions during discharge and absorbing ions such as lithium ions during charge.

[0059] Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, and lithium-transition metal composite oxides (e.g., Li4Ti5O 12 ), metal materials (tin, silicon), silicon-containing alloy-based negative electrode materials (e.g., Si 60 Sn 10 Ti 30 ), and lithium alloy-based negative electrode materials (for example, lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). In some cases, two or more types of negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, silicon-containing alloy-based negative electrode materials, carbon materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials are preferably used as the negative electrode active material. Of course, negative electrode active materials other than those mentioned above may also be used.

[0060] Average particle diameter of the negative electrode active material (D 50) is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm.

[0061] The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more and less than 100% by mass, preferably 80% by mass or more and 99.5% by mass or less, more preferably more than 95% by mass and 99.0% by mass or less, and even more preferably 97% by mass or more and 98.5% by mass or less, relative to 100% by mass of the total solid content. If the content of the negative electrode active material is within the above range, both battery capacity and output characteristics can be achieved.

[0062] Furthermore, the negative electrode active material layer may further contain other additives such as a conductive aid and a binder, as described above for the positive electrode active material layer, as needed.

[0063] The thickness of the negative electrode active material layer is not particularly limited, and the same thickness as that described above for the positive electrode active material layer can be employed.

[0064] [Electrolyte layer] The electrolyte layer preferably has a configuration in which a separator is impregnated with an electrolytic solution (liquid electrolyte).

[0065] (electrolyte) The electrolyte functions as a carrier of lithium ions and has a form in which a lithium salt is dissolved in a non-aqueous solvent.

[0066] Examples of non-aqueous solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the non-aqueous solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0067] Examples of lithium salts include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. Of these, the lithium salt is preferably Li(FSO2)2N from the viewpoints of battery output and charge / discharge cycle characteristics.

[0068] The concentration of the lithium salt in the electrolytic solution is preferably 0.1 to 3.0 mol / L, and more preferably 0.8 to 2.2 mol / L.

[0069] The electrolyte may further contain additives other than the above-mentioned components. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of the additive include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. The amount of additive used in the electrolyte solution can be adjusted as appropriate.

[0070] (separator) The separator constituting the electrolyte layer has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and also functions as a partition wall between the positive electrode and the negative electrode.

[0071] Examples of the form of the separator include a porous sheet separator made of polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

[0072] As a separator made of a porous sheet made of a polymer or fiber, for example, a microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of a polymer or fiber include microporous (microporous membrane) separators made of polyolefins such as polyethylene (PE) and polypropylene (PP), laminates of multiple layers of these (for example, a laminate with a three-layer structure of PP / PE / PP), hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and glass fibers.

[0073] The thickness of a microporous (microporous membrane) separator cannot be uniquely defined because it varies depending on the intended use. For example, in applications such as motor-driven secondary batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs), and fuel cell vehicles (FCVs), a single-layer or multi-layer thickness of 4 to 60 μm is desirable. The micropore diameter of the microporous (microporous membrane) separator is desirably a maximum of 1 μm or less (usually a pore diameter of about several tens of nanometers).

[0074] As the nonwoven fabric separator, conventional materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide, aramid, etc. may be used alone or in combination. The bulk density of the nonwoven fabric is not particularly limited as long as it can provide sufficient battery characteristics with the impregnated polymer gel electrolyte. The thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

[0075] Furthermore, the separator is preferably a separator in which a heat-resistant insulating layer is laminated on a porous substrate (a separator with a heat-resistant insulating layer). The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. The separator with a heat-resistant insulating layer is highly heat-resistant, with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. The presence of the heat-resistant insulating layer alleviates the internal stress of the separator that increases with temperature rise, thereby suppressing thermal shrinkage. As a result, short circuits between battery electrodes can be prevented, resulting in a battery configuration that is less susceptible to performance degradation due to temperature rise. Furthermore, the presence of the heat-resistant insulating layer improves the mechanical strength of the separator with a heat-resistant insulating layer, making it less likely to rupture. Furthermore, the heat-shrinkage suppression effect and high mechanical strength make the separator less likely to curl during the battery manufacturing process.

[0076] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for lithium-ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 may be made of the same material or different materials.

[0077] [Positive and negative leads] Although not shown, the current collector 11 and the current collector plates (25, 27) may be electrically connected via a positive electrode lead and a negative electrode lead. The materials used in known lithium-ion secondary batteries may be used as the constituent materials of the positive and negative electrode leads. It is preferable that the portion removed from the outer casing be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting the product (e.g., automobile parts, particularly electronic devices).

[0078] [Sealing part] The sealing portion (insulating layer) 31 is a component specific to bipolar secondary batteries (series-stacked batteries) and functions to prevent leakage of the electrolyte solution from the electrolyte layer. The sealing portion 31 also functions to prevent contact between current collectors and short-circuiting at the ends of the cell layers. The sealing portion may be made of any material that has insulating properties, sealing properties against the loss of the solid electrolyte and against moisture penetration from the outside, and heat resistance at the battery operating temperature. Examples of materials that can be used include acrylic resin, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, and rubber (ethylene-propylene-diene rubber: EPDM). Other suitable materials include isocyanate-based adhesives, acrylic resin-based adhesives, and cyanoacrylate-based adhesives, as well as hot-melt adhesives (urethane resin, polyamide resin, polyolefin resin). Among these, polyethylene resin and polypropylene resin are preferably used as the constituent material of the sealing portion from the viewpoints of corrosion resistance, chemical resistance, ease of production (film-forming ability), economy, etc., and it is more preferable to use a resin copolymerized with ethylene, propylene, and butene, with amorphous polypropylene resin as the main component.

[0079] [Battery exterior] As the battery exterior, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element, as shown in Figures 1 and 2, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited to these. A laminate film is desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminate laminate is more preferable for the exterior because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside and allows for easy adjustment of the electrolyte layer thickness to the desired value.

[0080] The lithium ion secondary battery according to this embodiment can exhibit excellent output characteristics, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0081] [Battery pack] A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

[0082] A small, detachable assembled battery can be formed by connecting multiple batteries in series or in parallel. Furthermore, a large-capacity, high-output assembled battery (such as a battery module or battery pack) can be formed by further connecting multiple such small, detachable assembled batteries in series or in parallel, suitable for use as a vehicle drive power source or auxiliary power source, which require high volumetric energy density and high volumetric power density. The number of batteries to be connected to form a battery assembly and the number of stacked small assembled batteries to form a large-capacity assembled battery can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which the battery will be installed.

[0083] [vehicle] The lithium ion secondary battery of this embodiment has excellent output characteristics. Furthermore, it has a high volumetric energy density. In vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles, higher capacity and larger current are required compared to applications in electrical and portable electronic devices. Therefore, the lithium ion secondary battery can be suitably used as a power source for vehicles, for example, as a vehicle drive power source or auxiliary power source.

[0084] Specifically, a battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. Since the present invention can provide a battery with large capacity and excellent output characteristics, mounting such a battery on a vehicle can provide a plug-in hybrid electric vehicle with a long EV driving range or an electric vehicle with a long driving range per charge. Examples of vehicles include hybrid vehicles, fuel cell vehicles, and electric vehicles (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles). However, the application is not limited to automobiles, and the battery can also be applied to various power sources for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like. [Example]

[0085] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0086] <Cathode active material> The following two types of commercially available positive electrode active materials were prepared.

[0087] Single crystal NMC composite oxide (Easpring, ME88SC, LiNi 0.86 Mn 0.08 Co 0.06 O2, roughness factor: 2.4, crystallite size (X): 110 nm, average particle size (D50) (Y): 4000 nm, Y / X: 36.4, crystal structure: space group R3m, peak intensity ratio (003) / (104): 1.48, tap density: 1.76 g / cm 3 ).

[0088] Polycrystalline NMC composite oxide (manufactured by Ecopro, NMC811, LiNi 0.8 Mn 0.1 Co 0.1O2, roughness factor: 16.5, crystallite diameter (X): 80 nm, average particle diameter (D50) (Y): 11,500 nm, Y / X: 144, crystal structure: space group R3m, peak intensity ratio (003) / (104): 1.29, tap density: 2.68 g / cm 3 ).

[0089] The physical properties of each positive electrode active material were measured by the following methods.

[0090] [Roughness Factor] First, the powder of the positive electrode active material was observed using a scanning electron microscope (SEM). From the obtained SEM image, the maximum distance between any two points on the outline of the active material particle was measured as the particle diameter, and the arithmetic mean value of the particle diameters of particles observed within several dozen fields of view was calculated as the average particle diameter.

[0091] Next, from the value of the average particle diameter calculated above, the volume [m 3 ] and particle surface area [m 2 ] was calculated, and the specific gravity of the active material (here, 4.78 [g / cm 3 The mass [g] of the spherical particles was calculated by multiplying the surface area of the particles calculated above by the mass of the particles also calculated above. The geometric specific surface area [m 2 / g] was calculated.

[0092] On the other hand, in accordance with the "Method for measuring the specific surface area of powders (solids) by gas adsorption" described in JIS Z8830:2013 (ISO 9277:2010), measurements were performed using nitrogen gas as the adsorption gas by the static volume method, and the BET specific surface area [m 2 / g] was calculated. The roughness factor was calculated as the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle size" calculated above (=BET specific surface area / geometric specific surface area).

[0093] [Y / X] (Crystallite diameter X) The crystallite diameter X of the positive electrode active material was calculated using the Williamson-Hall method (Williamson-Hall method, Hall, W.II., J. Inst. Met., 75, 1127 (1950); iDem, Proc. Phys. Soc., A62, 741 (1949)). An X-ray diffraction (XRD) device (manufactured by Rigaku) using CuKα radiation was used for the measurement. According to the Williamson-Hall method, the following relationship holds between the θ of the diffraction peak obtained by X-ray diffraction measurement of the positive electrode active material and its integral value β:

[0094] βcosθ / λ=2η(sinθ / λ)+(1 / ε) Here, ε is the crystallite diameter, λ is the wavelength of CuKα radiation, and η is the distortion of the positive electrode active material. With sinθ on the X axis and βcosθ on the Y axis, the θ of the diffraction peak of the positive electrode active material and its integral width β are plotted on the X and Y axes, and an approximate line is drawn using the least squares method. The crystallite size ε is calculated from the point where it intersects with the Y axis.

[0095] (Average particle diameter (D50)) The average particle size (D50) of the positive electrode active material was measured by laser diffraction, where D50 represents the particle size when the cumulative value of the particle size distribution on a volume basis is 50%.

[0096] [Crystal structure, peak intensity ratio (003) / (104)] Powder X-ray diffraction measurements to calculate the crystal structure and peak intensity ratio (I(003) / I(104)) of the positive electrode active material were performed using an X-ray diffractometer (Rigaku) with CuKα radiation, and analysis was performed using fundamental parameters. X-ray diffraction patterns obtained from a diffraction angle range of 2θ = 15 to 120° were analyzed using the analysis software Topas Version 3.

[0097] [Tap Density] The positive electrode active material was placed in a 10 mL glass measuring cylinder, and the powder packing density after tapping 200 times was measured, and this was taken as the tap density of the positive electrode active material.

[0098] <Preparation of positive electrode> [Example 1] A solid content was prepared consisting of 97 parts by mass (97.98% by mass) of the above single-crystal NMC composite oxide as the positive electrode active material, 1 part by mass (1.01% by mass) of carbon nanotubes (abbreviated as CNT, fiber diameter: 10 nm, fiber length: 20 μm, aspect ratio: 2000) as a conductive additive, and 1 part by mass (1.01% by mass) of PVDF-HFP (Arkema, Kyner Flex 2501, proportion of hexafluoropropylene-derived structural units: 6.9 mol%) as a binder. N-methyl-2-pyrrolidone (NMP) as a solvent was added to this solid content and mixed to obtain a positive electrode slurry with a solid content concentration of 70% by mass. The weight per unit area was 2.5 g / cm. 3 The positive electrode slurry was applied to a 20 μm thick aluminum foil using a doctor blade and a coater (manufactured by Tester Sangyo) with the gap adjusted so that the positive electrode slurry was applied to the aluminum foil. The coating was then dried for 1 hour on a hot plate adjusted to 80 ° C. The dried coating was pressed using a small desktop roll press (manufactured by Tester Sangyo) to a porosity of 20%. Then, the positive electrode of this example was obtained by placing it in a vacuum dryer and drying it at 130 ° C. for 8 hours under vacuum.

[0099] [Example 2] The positive electrode of this example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 2 parts by mass (2.00% by mass).

[0100] [Example 3] The positive electrode of this example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 3 parts by mass (2.97% by mass).

[0101] [Comparative Example 1] The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 0.3 parts by mass (0.31% by mass).

[0102] Comparative Example 2 The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the blending amount of PVDF-HFP as a binder was set to 5 parts by mass (4.85% by mass).

[0103] Comparative Example 3 The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 97 parts by mass (97.98 mass%) of the polycrystalline NMC composite oxide was used as the positive electrode active material.

[0104] Comparative Example 4 The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was set to 2 parts by mass (2.00% by mass).

[0105] Comparative Example 5 The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was 3 parts by mass (2.97% by mass).

[0106] Comparative Example 6 The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the blending amount of PVDF-HFP as a binder was set to 5 parts by mass (4.85% by mass).

[0107] Comparative Example 7 The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

[0108] [Comparative Example 8] The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 2 parts by mass (2.00% by mass).

[0109] Comparative Example 9 The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 3 parts by mass (2.97% by mass).

[0110] [Comparative Example 10] The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the blending amount of PVDF as a binder was set to 5 parts by mass (4.85% by mass).

[0111] [Comparative Example 11] The positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 3 parts by mass (2.97 mass%) of PVDF-HFP (Kyner Flex 2851, manufactured by Arkema, proportion of the number of structural units derived from hexafluoropropylene: 2.4 mol%) was used as the binder.

[0112] [Comparative Example 12] The positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

[0113] <Preparation of negative electrode> A solid content consisting of 95.5% by mass of graphite (average particle size: 20 μm) as the negative electrode active material, 0.5% by mass of acetylene black as the conductive additive, and 4% by mass of PVDF as the binder was prepared. N-methyl-2-pyrrolidone (NMP) was added to this solid content and mixed to obtain a negative electrode slurry with a solid content concentration of 70%. The basis weight was 2.5 g / cm. 3 The negative electrode slurry was applied to a 20 μm thick copper foil using a doctor blade and a coater (manufactured by Tester Sangyo) with the gap adjusted to 100 μm. The coating was then dried for 1 hour on a hot plate adjusted to 80 ° C. The dried coating was pressed using a small tabletop roll press (manufactured by Tester Sangyo) to achieve a porosity of 20%. The resultant was then placed in a vacuum dryer and dried at 130 ° C. for 8 hours under vacuum conditions to obtain a negative electrode.

[0114] <Fabrication of lithium-ion secondary batteries> The resulting positive electrode was placed in a 12cm 2 , negative electrode 13cm 2The cathode was cut into pieces so that they resembled a rectangular parallelepiped. For the cathode, an aluminum foil with an Al terminal was laminated on the collector foil side of the cut cathode. For the anode, a copper foil with a Ni terminal was laminated on the collector foil side of the cut anode. Separators (Celgard, PP) were inserted on the electrode active material layer sides of the cathode and anode to form a laminate. This laminate was sandwiched between heat-sealable aluminum laminate films, and three sides were sealed. 280 μL of electrolyte obtained by dissolving 2 mol / L of Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI) in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 3:7 (EC:EMC)) was injected from the remaining side, and the battery was vacuum-sealed at a vacuum level of 99.7% using a vacuum sealer (TOSPACK). In this way, a pouch-type lithium-ion battery was fabricated, in which the cathode and anode were stacked facing each other with the separator interposed between them.

[0115] [Measurement of DC resistance (DCR)] At 25°C, the initial and second charge / discharge cycles were performed at 0.1C (1C = 220 mAh / g) in a charge / CV mode with a voltage range of 2.5 to 4.6V, and in discharge / CC mode (with an 8-hour break between charge and discharge). Discharge was performed at 0.1C, 0.5C, and 1C at an SOC of 50% (voltage of 3.9V), and the direct current resistance (DCR) values (Ωcm) were calculated from the current values (1 sec, 10 sec, and 30 sec) flowing during discharge. The DCR value in Comparative Example 12 was designated X0, and the DCR value in each example was designated X1. The percentage decrease in X1 relative to X0 was calculated using the formula: {(X0 - X1) / X0} × 100.

[0116] The results are shown in Table 1 below.

[0117] [Table 1]

[0118] The results in Table 1 show that the positive electrode for a lithium ion secondary battery of the present invention significantly reduces the direct current resistance (DCR) compared to Comparative Example 12. Therefore, by applying this positive electrode to a lithium ion secondary battery, it is possible to improve the output characteristics of the lithium ion secondary battery.

[0119] On the other hand, in Comparative Example 1, the structure of the positive electrode active material layer could not be maintained due to the low binder content, and electrode cracking occurred. In Comparative Example 2, the DC resistance increased due to the high binder content.

[0120] In the examples using polycrystalline NMC composite oxides in Comparative Examples 3 to 5, no effect of reducing DC resistance was observed compared to Comparative Example 12, or the reduction was slight.

[0121] In the examples using PVDF in Comparative Examples 7 to 9 and the example using PVDF-HFP with a low HFP ratio in Comparative Example 11, a reduction in DC resistance was observed compared to Comparative Example 12, but the extent of the reduction was not sufficient. [Explanation of symbols]

[0122] 10a stacked secondary battery, 10b Bipolar secondary battery; 11 current collector, 11a: outermost current collector on the positive electrode side; 11b: outermost current collector on the negative electrode side; 11' Positive electrode current collector 12 Negative electrode current collector 13 positive electrode active material layer, 15 negative electrode active material layer, 17 electrolyte layer, 19 cell layer, 21 power generation elements, 23 Bipolar electrodes, 25 Positive current collector plate (positive tab), 27 negative electrode current collector plate (negative electrode tab), 29 Laminating film, 31 Seal part.

Claims

1. A positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, a binder, and a conductive additive, the positive electrode active material has a roughness factor of 3 or less, which is defined as the ratio of a BET specific surface area to a geometric specific surface area calculated from an average particle size; the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and in this case, the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more; the content of the binder is greater than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer, The positive electrode active material satisfies Y / X≦70, where X (nm) is the crystallite diameter calculated by the Williamson-Hall method and Y (nm) is the average particle diameter (D50) calculated by a laser diffraction method.

2. A positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, a binder, and a conductive additive, the positive electrode active material has a roughness factor of 3 or less, which is defined as the ratio of a BET specific surface area to a geometric specific surface area calculated from an average particle size; the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and in this case, the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more; the content of the binder is greater than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer, the positive electrode active material is composed of a space group of R3m, and has a peak intensity ratio (003) / (104) of a diffraction peak of a (003) plane to a diffraction peak of a (104) plane obtained by X-ray diffraction measurement of 1.35 or more.

3. The tap density of the positive electrode active material is 2 g / cm ３ The positive electrode for a lithium ion secondary battery according to claim 1 or 2, wherein:

4. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure.

5. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the conductive additive is a carbon nanotube.

6. A lithium ion secondary battery comprising a power generating element having the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5.

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